Methods such as these are used, for example, in an apparatus for reading from and/or writing to optical recording media having wobbled tracks, in order to obtain address information from the wobbled tracks (ADIP information, address in pregroove) or to use the wobble frequency to produce a write clock.
In general, in optical recording media, which are in the form of discs and are suitable for reading from and/or writing to, the tracks are formed such that they represent an interleaved spiral or concentric circles. Especially in the case of optical recording media which are suitable for writing to, the tracks additionally are wobbled in a specific form, in order to find specific positions on the medium. This means that the track is not an approximately straight line, but a serpentine line. By way of example, the shape of this serpentine line can contain address information, which is used for identifying a specific position on this optical recording medium. Various methods are used for coding, examples of which include frequency modulation or phase modulation. Furthermore, the wobble signal may also be used for rotation speed information or for presetting a write data rate.
For high density optical recording media, it has been proposed to modulate the wobble signal using two methods in an intermixed manner: Minimum Shift Keying cosine variant (MSK-cos) and Harmonic Modulated Wave (HMW), which is also referred to as sawtooth wobble. Only some of the wobble periods are modulated. Most of the wobble periods are monotone wobbles (MW), as depicted in FIG. 1a). The MSK-cos method is mainly adopted for the ADIP unit synchronization, and is illustrated in FIG. 2a). The MSK mark indicates the start of the ADIP unit or is used for synchronization or data recognition. The HMW method is mainly employed for the ADIP data. The second harmonic of the fundamental wobble frequency is added to the wobble with a lower amplitude level. Its phase is in quadrature with the fundamental wobble frequency and it is bi-phase modulated according to the ADIP bit, which is illustrated in FIGS. 3 and 4.
In the wobble signal of the proposed high density optical recording medium the following frequencies with different phases occur.f1(t)=cos(2·π·fwob·t)f2(t)=−cos(2·π·fwob·t)=−f1(t)f3(t)=cos(2·π·1.5·fwob·t)f4(t)=−cos(2·π·1.5·fwob·t)=−f3(t)f5(t)=cos(2πfwob·t)+¼·sin(2·π·2·fwob)·t)f6(t)=cos(2·πfwob·t)−¼·sin(2·π2·fwob)·t)
As can be seen from the above list, depending on the frequency and the phase of the wobble signal different types of wobble periods are found in the wobble signal.
Since the above described modulation of the wobble signal is quite new, solutions for a reliable wobble demodulation are hardly known. Typical schemes known from prior art for frequency or phase demodulation could be used, but it is difficult to apply the proper combination of both schemes.
Minamino et al. in Jpn. J. Appl. Phys Vol. 41 (2002), pp 1741-1742, propose a new concept of addressing in optical disks using a sawtooth wobble groove. For calculating the error rate of every sawtooth wobble the steep edges of the sawtooth shape are converted to pulse signals by differential calculus.
Kobayashi et al. in Jpn. J. Appl. Phys Vol. 42 (2003), pp 915-918, propose a method for detecting the MSK marks and the HMW sawtooth wobble. A heterodyne circuit consisting of a carrier multiplier, an integrator and a sample and hold element is used for this purpose. The wobble signal is multiplied by the cosine carrier of the fundamental frequency for detecting the MSK marks in the multiplier. On the other hand it is multiplied by the sine carrier of the second harmonic frequency for detecting the HMW sawtooth wobble.
It is an object of the invention to propose an alternative method for a reliable wobble demodulation.